refactor: replace standard log with zap logger and optimize code patterns

- Migrate all log.Printf/log.Println calls to structured zap logging
- Use cmp.Or for cleaner default value handling
- Replace manual loops with slices.ContainsFunc/IndexFunc
- Add batch query methods (IsLikedBatch, IsFavoritedBatch) to solve N+1 problem
- Update JSON tags from omitempty to omitzero for numeric fields
- Use errgroup-style wg.Go for worker goroutines
- Remove duplicate casbin/v2 dependency (keeping v3)
- Add plans/ to gitignore

internal/cache/conversation_cache.go

@@ -4,10 +4,11 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"log"
 	"time"
 
 	"carrot_bbs/internal/model"
+
+	"go.uber.org/zap"
 )
 
 // CachedConversation 带缓存元数据的会话
@@ -713,6 +714,10 @@ func (c *ConversationCache) getMessagesBySeqs(ctx context.Context, convID string
 // asyncCacheMessage 异步缓存单条消息
 func (c *ConversationCache) asyncCacheMessage(ctx context.Context, convID string, msg *model.Message) {
 	if err := c.CacheMessage(ctx, convID, msg); err != nil {
-		log.Printf("[ConversationCache] async cache message failed, convID=%s, msgID=%s, err=%v", convID, msg.ID, err)
+		zap.L().Error("async cache message failed",
+			zap.String("convID", convID),
+			zap.String("msgID", msg.ID),
+			zap.Error(err),
+		)
 	}
 }